Digit, a popular app that automates savings for its users, will soon begin charging a monthly fee in what will amount to a major test of its Millennial fan base.

The app, which monitors the activity in your bank account and diverts small amounts into savings, has decided to start charging $2.99 a month. There will be a 100-day free trial that will give users a chance to decide if the app is worth paying for or not. The clock starts ticking for existing users on April 11.

"This is the year that Digit is growing up," said CEO Ethan Bloch in a phone interview. "We're going to continue to prove we can do more for the customer and continue to prove that we can build a long-term sustainable business."